Semiconductor light emitting element and method for fabricating the same

ABSTRACT

The semiconductor laser of this invention includes an active layer formed in a c-axis direction, wherein the active layer is made of a hexagonal-system compound semiconductor, and anisotropic strain is generated in a c plane of the active layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a short-wavelength semiconductor lightemitting element used in the fields of optical communications, opticalinformation processing, and the like, and a method for fabricating thesame.

2. Description of the Related Art

In recent years, with increased demands for short-wavelengthsemiconductor light emitting elements in various fields, studiesfocusing mainly on ZnSe and GaN as the materials for such elements havebeen vigorously conducted. As for ZnSe material, a short-wavelengthsemiconductor laser with an oscillation wavelength of about 500 nm hassucceeded in oscillating consecutively at room temperature. Now, studyand development for practical use of this material is under way. As forGaN material, a blue light emitting diode with high luminance hasrecently been realized. The reliability of this material as the lightemitting diode is by no means inferior to that of other materials forsemiconductor light emitting elements. GaN material is thereforeexpected to be applicable to a semiconductor laser.

However, the properties of GaN material are not clearly known; moreover,GaN material has a hexagonal-system crystalline structure. Therefore, itis uncertain whether GaN material can provide characteristics durableenough for practical use when it is used as an element having astructure similar to that used for conventional cubic-system materials.

SUMMARY OF THE INVENTION

The semiconductor laser of this invention includes an active layerformed in a c-axis direction, wherein the active layer is made of ahexagonal-system compound semiconductor and anisotropic strain isgenerated in a c plane of the active layer.

In another aspect of the present invention, a method for fabricating asemiconductor laser is provided. The method includes the step of formingan active layer made of a hexagonal-system compound semiconductor in ac-axis direction, wherein the active layer is formed so that anisotropicstrain is generated in a c plane.

Alternatively, the semiconductor light emitting element of this exampleincludes: a semiconductor substrate; a stripe groove formed on aprincipal plane of the semiconductor substrate; and a semiconductorlight emitting layer formed on the other principal plane of thesemiconductor substrate.

Alternatively, the method for fabricating a semiconductor light emittingelement of this example includes the steps of: forming a stripe-shapedgroove on a principal plane of a semiconductor substrate; and forming alight emitting element structure on the other principal plane of thesemiconductor substrate.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: forming a stripe-shapedmask on a principal plane of a semiconductor substrate; etching thesemiconductor substrate selectively using the mask; depositing materialhaving a thermal expansion coefficient different from that of thesemiconductor substrate on the semiconductor substrate selectively usingthe mask; and forming a light emitting element structure on the otherprincipal plane of the semiconductor substrate.

Alternatively, the semiconductor light emitting element of this exampleincludes: a semiconductor substrate; a stripe-shaped member formed on aprincipal plane of the semiconductor substrate, the member being made ofa material having a thermal expansion coefficient different from that ofthe semiconductor substrate; and a semiconductor light emitting layerformed on the other principal plane of the semiconductor substrate.

Alternatively, the method for fabricating a semiconductor light emittingelement of this example includes the steps of: forming a stripe-shapedmember on a principal plane of a semiconductor substrate, the memberbeing made of a material having a thermal expansion coefficientdifferent from that of the semiconductor substrate; and forming a lightemitting element structure on the other principal plane of thesemiconductor substrate.

Alternatively, the method for fabricating a semiconductor light emittingelement of this example includes the steps of: forming a light emittingelement structure on a surface of a semiconductor substrate; and forminga stripe-shaped member on the other surface of the semiconductorsubstrate at 300° C. or more, the member being made of a material havinga thermal expansion coefficient different from that of the semiconductorsubstrate.

Alternatively, the method for fabricating a semiconductor light emittingelement of this example includes the steps of: forming a light emittingelement structure on a principal plane of a semiconductor substrate;forming a stripe-shaped member on the other surface of the semiconductorsubstrate, the member being made of a material having a thermalexpansion coefficient different from that of the semiconductorsubstrate; and heat-treating the semiconductor substrate at 500° C. ormore.

Alternatively, the semiconductor light emitting element of this exampleincludes: a semiconductor substrate; a first metal formed on a principalplane of the semiconductor substrate; a stripe-shaped second metalformed on the first metal; and a light emitting element structure formedon the semiconductor substrate.

Alternatively, the method for fabricating the semiconductor lightemitting element of this example includes the steps of: forming a lightemitting element structure on a principal plane of a semiconductorsubstrate; depositing a first metal on the other principal plane of thesemiconductor substrate; and depositing a stripe-shaped second metal onthe first metal.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: attaching asemiconductor substrate to a surface of a body which is part of a curvedsurface of a cylinder; and forming a light emitting element structure onthe semiconductor substrate.

Alternatively, the semiconductor light emitting element of thisinvention includes: a substrate having a principal plane; and awurtzite-type AlGaInN compound semiconductor formed on the substrate,wherein the substrate is made of a material of which thermal expansioncoefficient is anisotropic in the principal plane.

Alternatively, the semiconductor light emitting element of thisinvention includes a substrate having a principal plane and awurtzite-type AlGaInN compound semiconductor formed on the substrate,wherein the substrate is made of a material of which thermal expansioncoefficient is greater in a first direction in the principal plane andsmaller in a second direction vertical to the first direction than thethermal expansion coefficient of the wurtzite-type AlGaInN compoundsemiconductor.

Alternatively, the semiconductor light emitting element of thisinvention includes a wurtzite-type AlGaInN compound semiconductor wherea total of a thermal strain in a first direction in a substrate planeand a thermal strain in a second direction vertical to the firstdirection generated when the element is cooled from a growth temperatureto room temperature is zero.

Alternatively, the semiconductor light emitting element of thisinvention includes: an active layer made of a wurtzite-type compoundsemiconductor; a pair of carrier confinement layers sandwiching theactive layer; and a stripe-shaped strain generating layer having alattice constant different from that of the pair of carrier confinementlayers.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: placing a semiconductorlight emitting element having a double-hetero structure on ananisotropic crystal; and securing the semiconductor light emittingelement to the anisotropic crystal at 100° C. or more.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: placing a semiconductorlight emitting element having a double-hetero structure on a bimetal;and securing the semiconductor light emitting element to the bimetal at100° C. or more.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: placing a semiconductorlight emitting element having a double-hetero structure on a sub-mount;applying stress to the semiconductor light emitting element from a topsurface or a side face thereof; and securing the semiconductor lightemitting element to the sub-mount.

Alternatively, the method for fabricating an AlGaInN semiconductor lightemitting element of this invention including a substrate having a stepand an AlGaInN double-hetero structure formed on the substrate isprovided. The method includes the steps of: forming at least two stripgrooves on an AlGaInN thin film to obtain a mesa structure; and forminga multilayer structure including the AlGaInN double-hetero structure onthe entire top surface of the substrate including the inside of the atleast two stripe grooves so that a crystal mixture ratio of AlGaInN on aflat surface of the mesa structure is different from that on a slopesurface of the mesa structure.

Alternatively, the method for fabricating an AlGaInN semiconductor lightemitting element of this invention including a substrate having a stepand an AlGaInN double-hetero structure formed on the substrate. Themethod comprising the steps of: forming a stripe groove on an AlGaInNthin film to obtain a concave groove structure; and forming a multilayerstructure including the AlGaInN double-hetero structure on the entiretop surface of the substrate including the inside of the stripe grooveso that a crystal mixture ratio of AlGaInN on a flat surface of theconcave groove structure is different from that on a slope surface ofthe concave groove structure.

Alternatively, the method for fabricating a nitride compoundsemiconductor of this invention includes the step of forming a nitridecompound semiconductor by vapor phase epitaxy while selectivelyirradiating the nitride compound semiconductor, so as to form anirradiated portion and a non-irradiated portion having different latticeconstants.

Alternatively, the method for fabricating a nitride compoundsemiconductor of this invention includes the steps of: forming a nitridecompound semiconductor by vapor phase epitaxy while selectivelyirradiating the nitride compound semiconductor, so as to form anirradiated portion and a non-irradiated portion having different latticeconstants; and forming a nitride compound semiconductor by vapor phaseepitaxy at a temperature higher than a temperature used for the formergrowth step.

Alternatively, the semiconductor light emitting element of thisinvention includes: a substrate; a first cladding layer formed on thesubstrate, an area of a plane parallel to the substrate being smallerthan an area of a surface of the substrate; a second cladding layerformed on the first cladding layer, an area of a plane parallel to thesubstrate being larger than the area of the first cladding layer, thesecond cladding layer being made of crystal having a lattice constantdifferent from that of the first cladding layer; an active layer formedon the second cladding layer; and a third cladding layer formed on theactive layer.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: forming a firstcladding layer on a substrate; forming a second cladding layer on thefirst cladding layer; forming an active layer on the second claddinglayer; forming a third cladding layer on the active layer; and etchingso that the first cladding layer can be etched faster than thesubstrate, the second cladding layer, the active layer, and the thirdcladding layer.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: forming a firstcladding layer on a substrate; forming a second cladding layer on thefirst cladding layer; forming an active layer on the second claddinglayer; forming a third cladding layer on the active layer; forming aninsulating film on faces of the substrate, the first cladding layer, thesecond cladding layer, the active layer, and the third cladding layervertical to a depositing direction; removing a portion of the insulatingfilm so as to expose the side face of the first cladding layer; andetching so that the first cladding layer can be etched faster than theinsulating film.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: forming a firstconductive semiconductor on a substrate; forming an insulatingsemiconductor on the first conductive semiconductor, the insulatingsemiconductor having a lattice constant different from that of the firstconductive semiconductor; forming a semiconductor layer of adouble-hetero structure on the insulating semiconductor; and etching thefirst conductive semiconductor by immersing the substrate, the firstconductive semiconductor, and the insulating semiconductor in anelectrolytic solution and attaching a positive electrode and a negativeelectrode to the first conductive semiconductor or the insulatingsemiconductor for applying a voltage between the electrodes.

Alternatively, the semiconductor light emitting element of thisinvention includes: a substrate; a semiconductor crystal nucleusdeposited on the substrate; a thin film spirally formed around thecrystal nucleus in parallel to the substrate; a first cladding layerformed on the thin film; an active layer formed on the first claddinglayer; and a second cladding layer formed on the active layer.

Alternatively, the method for fabricating a semiconductor light emittingelement of this invention includes the steps of: forming a semiconductorcrystal nucleus on a substrate under a first pressure condition by vaporphase epitaxy; forming a thin film around the crystal nucleus spirallyin parallel to the substrate under a second pressure condition; forminga first cladding layer under a third pressure condition; forming anactive layer on the first cladding layer under the third pressurecondition; and forming a second cladding layer on the active layer underthe third pressure condition.

In still another aspect of the present invention, a semiconductor lightemitting device is provided. The device includes a base having a concaveportion and a semiconductor light emitting element formed in the concaveportion, wherein an active layer of the semiconductor light emittingelement is made of a hexagonal-system compound semiconductor, andanisotropic strain is generated in a c plane of the active layer due tostress from the base.

Alternatively, the semiconductor light emitting device of this inventionincludes a semiconductor light emitting element and a stress applyingportion for applying stress to an active layer of the semiconductorlight emitting element, wherein the active layer of the semiconductorlight emitting element is made of a hexagonal-system compoundsemiconductor, and anisotropic strain is applied to a c plane of theactive layer from the stress applying portion.

In still another aspect of the present invention, an epitaxial methodfor epitaxially growing crystal on a substrate causing latticemismatching is provided. In the method, lattice strain generated in anepitaxial layer due to the lattice mismatching between crystals of thesubstrate and the epitaxial layer is concentrated in a specificdirection of the epitaxial layer, so as to generate anisotropic strainin the epitaxial layer.

Thus, the invention described herein makes possible the advantages of(1) providing a semiconductor light emitting element with highperformance and a simple structure where the strain characteristic of anelectronic band structure unique to a hexagonal-system compoundsemiconductor is utilized, i.e., providing a semiconductor lightemitting element with a low threshold current by applying anisotropicstrain to the c plane of a hexagonal-system compound semiconductor, and(2) providing a method for fabricating such a semiconductor lightemitting element.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electronic band structure of a valence band of a GaNlayer of an AlGaN/GaN quantum well structure.

FIG. 2 shows an electronic band structure of a valence band of a GaNlayer of an AlGaN/GaN quantum well structure when anisotropic strain isapplied to the c plane.

FIG. 3 shows the strain dependency of the threshold current density whenanisotropic strain is applied to the c plane.

FIG. 4 shows a semiconductor light emitting element of Example 1according to the present invention.

FIGS. 5A to 5E are sectional views showing a fabrication process of thesemiconductor light emitting element of Example 1 according to thepresent invention.

FIGS. 6A and 6B show another example of the semiconductor light emittingelement according to the present invention.

FIG. 7 shows still another example of the semiconductor light emittingelement according to the present invention.

FIG. 8 shows still another example of the semiconductor light emittingelement according to the present invention.

FIGS. 9A and 9B show a fabrication process of still another example ofthe semiconductor light emitting element according to the presentinvention.

FIG. 10 is a sectional view of a wurtzite-type InGaN/AlGaN quantum wellsemiconductor laser of Example 2 according to the present invention.

FIG. 11 shows two crystal plane directions perpendicular to each otheron a LiTaO₃ substrate.

FIG. 12 shows the state where the LiTaO₃ substrate is inclined from the(1100) plane in the (0001) or (1120) direction.

FIG. 13 is a sectional view of a wurtzite-type InGaN/AlGaN quantum wellsemiconductor laser of Example 3 according to the present invention.

FIG. 14 shows strain locally generated in the vicinity of a p-Al₂,Ga_(1-z), N strain generating layer.

FIG. 15 shows the thermal expansion coefficients of LiTaO₃ and GaN.

FIGS. 16A and 16B are perspective views showing a fabrication process ofa semiconductor laser of Example 4 according to the present invention.

FIG. 17 shows a sub-mount for the semiconductor laser of Example 4.

FIGS. 18A and 18B show a fabrication process of a semiconductor laser ofExample 5 according to the present invention.

FIG. 19 shows a sub-mount for the semiconductor laser of Example 5.

FIGS. 20A and 20B show another sub-mount for the semiconductor laser ofExample 5.

FIG. 21 is a sectional view showing a fabrication process of asemiconductor laser according to the present invention.

FIG. 22 is a sectional view showing another fabrication process of asemiconductor laser according to the present invention.

FIGS. 23A to 23C show a fabrication process of an AlGaInN semiconductorlight emitting element of Example 6 according to the present invention.

FIGS. 24A to 24C show a fabrication process of an AlGaInN semiconductorlight emitting element of Example 7 according to the present invention.

FIGS. 25A and 25B show a vapor phase epitaxy step including selectivelaser irradiation and the relationship between the lattice constant ofGaN crystal grown by the vapor phase epitaxy and the selective laserirradiation, respectively, of Example 8.

FIG. 26 shows the relationship between the laser irradiation intensityand the GaN lattice constant.

FIG. 27 shows a vapor phase epitaxy step including selective laserirradiation of Example 10.

FIGS. 28A to 28D are sectional views showing a fabrication process of asemiconductor device of Example 11 according to the present invention.

FIG. 29 is a schematic sectional view of the semiconductor device ofExample 11, together with a crystal structure of an active layer of thesemiconductor device.

FIGS. 30A to 30C schematically show a fabrication process of asemiconductor device of Example 12 according to the present invention.

FIG. 31 schematically shows a unit cell of a GaN spiral thin film ofExample 12.

FIGS. 32A and 32B are perspective views of a semiconductor lightemitting device of Example 14 according to the present invention.

FIGS. 33A and 33B are sectional views of a semiconductor light emittingdevice of Example 15 according to the present invention.

FIG. 34 is a sectional view of a crystal growth apparatus used inExample 13 according to the present invention.

FIGS. 35A to 35D are sectional views showing a crystal growth process inExample 13.

FIG. 36 is a sectional view of epitaxial layers formed by two-stageepitaxy as a comparative example.

FIG. 37 is a perspective view of epitaxial layers in Example 13.

FIG. 38 is a sectional view of epitaxial layers in the case of using anSiO₂ film in Example 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors have found that the effective mass of holes near the topof a valence band is reduced when strain which is not isotropic(anisotropic strain) is applied to the c plane of a hexagonal-systemcompound semiconductor. Using this property, a semiconductor lightemitting element with a low threshold current can be realized byapplying anisotropic strain to the c plane of an active layer composedof a hexagonal-system compound semiconductor grown in a c-axisdirection. The "isotropic" strain as used herein refers to a strainapplied to the c plane hydrostatically (isotropically).

Hereinbelow, the strain characteristic of the electronic band structureof a valence band of a hexagonal-system compound semiconductor used inthe present invention will be described with reference to theaccompanying drawings.

FIG. 1 shows an electronic band structure of a valence band of a GaNquantum well layer of an AlGaN/GaN quantum well structure when no strainis applied. The quantum well structure is composed of an AlGaN barrierlayer and the GaN quantum well layer. The thickness of the GaN quantumwell layer is 4 nm. In FIG. 1, a curve a represents a first-level energyband of a heavy hole, a curve b a second-level energy band of a heavyhole, a curve c a first-level energy band of a light hole, and a curve da second-level energy band of a light hole.

It is observed from FIG. 1 that the effective mass of holes near the topof the valence band (near wave number 0) is significantly large comparedwith that of zincblende type compound semiconductors. Also, whenuniaxial strain in the c-axis direction or an isotropic (biaxial) strainin the c plane is applied to the GaN quantum well layer, the effectivemass of holes near the top of the valence band is almost the same asthat obtained when no strain is applied. The uniaxial strain in thec-axis direction refers to the case where strain is applied only in thec-axis direction of the hexagonal-system compound semiconductor. Thebiaxial strain in the c plane refers to the case where strains of anequal magnitude are applied along axes vertical to each other, which isalso referred to as the isotropic strain.

A deformation energy generated by the application of anisotropic strainin the c plane of a hexagonal-system compound semiconductor can beexpressed by D₅ (e_(xx) -e_(yy) =2ie_(xy)) where D₅ denotes thedeformation potential when anisotropic strain is applied in the c plane,e_(xx) and e_(yy) denote the strains in two directions perpendicular toeach other in the c plane, and e_(xy) denotes the shearing strain in thec plane.

FIG. 2 shows an electronic band structure of the valence band of the GaNquantum well layer of the AlGaN/GaN quantum well structure whenanisotropic strain having a deformation energy of 10 meV is applied inthe c plane. As is observed from FIG. 2, the "curvature of the valenceband" in the region of small wave numbers is reduced when anisotropicstrain is applied. This indicates that the effective mass of holes nearthe top of the valence band is considerably small compared with the casewhere no strain is applied. This means that the state density near thetop of the valence band decreases, and thus the injected current densityrequired for laser oscillation can be low. Accordingly, only a smalloscillation threshold current is required for a semiconductor laserhaving an active layer where anisotropic strain is applied to a quantumwell layer thereof.

FIG. 3 shows the strain dependency of the threshold current densityobserved when anisotropic strain is applied to the c plane. The X axisrepresents the deformation energy and the Y axis represents thethreshold current density standardized by a value obtained when nostrain is applied. The results obtained when the threshold gain isvaried are shown in FIG. 3. As is observed from FIG. 3, the thresholdcurrent density significantly decreases by the application ofanisotropic strain, irrespective of the threshold gain values.

Though the shearing strain in the c plane is not taken intoconsideration in FIG. 3, a similar effect to the above is observed whenshearing strain is applied to the c plane.

As described above with reference to FIGS. 1 to 3, it has been foundthat, as for the semiconductor light emitting element using ahexagonal-system compound semiconductor as an active layer, an elementrequiring only a small threshold current can be realized by applyinganisotropic strain to the active layer.

Now, the element where anisotropic strain is applied to a hexagonalcompound semiconductor and the method for fabricating the same accordingto the present invention will be described by way of examples.

(EXAMPLE 1)

The semiconductor light emitting element of Example 1 according to thepresent invention will be described with reference to FIG. 4. An AlGaInNmaterial which is a III-V group compound semiconductor is used as ahexagonal-system compound semiconductor. Strain is applied in adirection parallel to the c plane. When a wurtzite-type material is usedfor an AlGaInN light emitting layer 100, the band structure (valenceband) can be changed by applying uniaxial strain in a direction verticalto the (0001) axis (parallel to the c plane). As a result, thecharacteristics of the light emitting element improve as describedabove.

By forming stripe-shaped grooves 102 on a sapphire substrate 101 asshown in FIG. 4, the directivity of the thermal expansion coefficient isexhibited on the substrate 101. By this formation of the grooves 102, itis possible to apply uniaxial strain in the x direction shown in FIG. 4to the AlGaInN light emitting layer 100 grown on the surface of thesubstrate 101 opposite to the grooves 102. Using the light emittinglayer 100 as an active layer, a semiconductor light emitting elementwith a small threshold current can be realized.

The method for fabricating the semiconductor light emitting element ofExample 1 will be described with reference to FIGS. 5A to 5E.

First, a stripe-shaped mask 104 is formed on a principal plane of asapphire substrate 103. Then, the substrate 103 is etched with anetchant such as hot sulfuric acid using the mask 104 so as to formstripe-shaped grooves 105. Then, a material such as AlN is selectivelygrown on the substrate 103 using the mask 104 so as to form AlN buriedlayers 106 only in the grooves 105. As a result, the thermal expansioncoefficient distribution is generated in the thickness direction. Thismakes it possible to generate uniaxial strain in the substrate when anAlGaInN light emitting layer 107 is formed on the substrate by crystalgrowth at a high temperature equal to or more than 1000° C. in a laterstage. Metalorganic vapor phase epitaxy (MOVPE) is used for the crystalgrowth. An appropriate temperature for the crystal growth of AlGaInN isconsidered to be about 1100° C. When the temperature is lowered andresumes room temperature, the AlGaInN light emitting layer 107 is in thestate of having uniaxial strain.

The strain applied to the AlGaInN light emitting layer 107 can begreater when the AlN buried layers 106 are formed, because the formationof the buried layers increases the thermal expansion and thus improvesthe heat transfer from a heater. The absolute amount of the strain to beapplied can be controlled by varying the width and depth of the grooves105, so as to obtain an optimal structure for the light emittingelement.

An alternative method for applying uniaxial strain to crystal is to formstripe-shaped oxide films on a substrate. FIGS. 6A, 6B, and 7 show analternative example of the semiconductor light emitting elementaccording to the present invention.

Before crystal growth, stripe-shaped oxide films 109 are formed on aprincipal plane of a sapphire substrate 108.

When the temperature is raised to 1000° C. or more for the crystalgrowth, the substrate is curved in the z direction shown in FIG. 7 dueto the difference in the thermal expansion coefficient between thesapphire substrate 108 and the oxide films 109. The crystal growth isconducted while the curved state being maintained, and thereafter thetemperature is lowered to room temperature. As a result, crystal havingstrain in the z direction is obtained. In this case, the strain amountcan be controlled by the width and pitch of the stripes. For example,when a semiconductor laser is fabricated, the width and the pitch arepreferably 5 microns and 10 microns, respectively.

Alternatively, the AlGaInN light emitting layer can be first formed byMOVPE. Then, the stripe-shaped oxide layers 109 are formed at a hightemperature of about 500° C., so that a curve similar to the above canbe formed and thus uniaxial strain can be generated in the crystal.Alternatively, the stripe-shaped oxide films 109 can be heated to a hightemperature after the formation thereof.

A bimetal effect can be used to provide an effect similar to the above.FIG. 8 shows still another example of the semiconductor light emittingelement according to the present invention. It is effective to use SiCfor a substrate 112. An AlGaInN light emitting layer 116 is first formedon the SiC substrate 112 by MOVPE. An Ni layer 113 is formed on the SiCsubstrate 112, and then stripe-shaped first Au layers 114 are formed onthe Ni layer 113. The SiC substrate 112 is then curved in the zdirection by the bimetal effect, and thus uniaxial strain is applied tothe AlGaInN light emitting layer 116. In this case, the ohmiccharacteristic is exhibited by the Ni layer 113 to the SiC substrate112, which provides an advantageous effect to the resultantsemiconductor light emitting element.

FIGS. 9A and 9B show yet another example of the semiconductor lightemitting device according to the present invention. In this alternativeexample, stress is applied externally before the crystal growth. Asapphire substrate 117 is secured on a tray 119 having a curvature Rwith fixtures 118. A light emitting layer is formed on the substrate117, and then the substrate 117 is removed from the tray 119. While thesubstrate 117 gradually resumes the original shape, strain is applied tothe light emitting layer. According to this method, the strain amount tobe applied can be controlled by mechanically changing the curvature ofthe tray 119.

(EXAMPLE 2)

FIG. 10 is a sectional view of a wurtzite-type In_(x) Ga_(1-x) N/Al_(y)Ga_(1-y) N quantum well semiconductor laser of Example 2 according tothe present invention. In_(x) Ga_(1-x) N and Al_(y) Ga_(1-y) N are usedfor a quantum well layer and a barrier layer, respectively.

Referring to FIG. 10, an AlN buffer layer 202, an n-Al_(z) Ga_(1-z) Ncladding layer 203, an Al_(y) Ga_(1-y) N first optical guide layer 204,an In_(x) Ga_(1-x) N/GaN multiple quantum well active layer 205 (amultilayer structure of In_(x) Ga_(1-x) N quantum well layers and GaNquantum well layers), an Al_(y) Ga_(1-y) N second optical guide layer206, and a p-Al_(z) Ga_(1-z) N cladding layer 207 are consecutivelyformed in this order on a (1100) LiTaO₃ substrate 201 by MOVPE.

A ridge stripe 208 is formed by etching, and an Sio₂ insulating film 209is formed over the top surface of the resultant structure. Openings 210and 211 are formed at the SiO₂ insulating film 209 for currentinjection. Finally, an anode electrode 212 and a cathode electrode 213are formed.

The layers constituting the wurtzite-type InGaN/AlGaN quantum wellsemiconductor laser are formed at a temperature range of 800° to 1100°C., except for the AlN buffer layer 202, when grown by MOVPE, forexample, though the growth temperatures for the layers are oftendifferent from one another depending on the composition and material tobe used. Accordingly, when room temperature is resumed after the crystalgrowth process, strain is generated in the crystal due to the differencein the thermal expansion coefficient between the crystal and thesubstrate. The crystal growth for all the layers after thepolycrystalline AlN buffer layer 202 is conducted using the AlN bufferlayer 202 as a seed crystal. Accordingly, the difference in the latticeconstant between the (1100) LiTaO₃ substrate 201 and the other layershardly affect the strain. There may be the case where the latticeconstants are different among the hetero structure composed of then-Al_(z) Ga_(1-z) N cladding layer 203 to the p-Al_(z) Ga_(1-z) Ncladding layer 207, and this difference in the lattice constant mayaffect the strain. In such a case, however, suitable materials andthicknesses can be selected to prevent an occurrence of misfitdislocation and the like. However, the above-described strain due to thedifference in the thermal expansion coefficient cannot be prevented.This strain is therefore positively utilized in this example.

FIG. 15 shows the thermal expansion coefficients of the LiTaO₃ substrate201 and the wurtzite-type GaN crystal in the plane. Since the (1100)LiTaO₃ substrate is used in this example, the thermal expansioncoefficient is anisotropic in the plane, which is expressed by the(0001) direction and the (1120) direction vertical to the (0001)direction, as shown in FIG. 11. As for the wurtzite-type GaN material,crystal grows at the (0001) orientation regardless of the crystal planedirection of the substrate. Accordingly, each layer is formed verticallyto the (0001) direction. The thermal expansion coefficient of thewurtzite-type GaN material is isotropic in the (0001) plane. The thermalexpansion coefficient of GaN is 5.6×10⁻⁶. Materials of AlGaInN mixedcrystal of any composition have the thermal expansion coefficients nearthe above value. On the contrary, the LiTaO₃ substrate has a thermalexpansion coefficient of 1.2×10⁻⁶ which is smaller than that of GaN inthe (0001) direction. In the (1120) direction, however, it has a thermalexpansion coefficient of 2.2×10⁻⁵ which is extremely larger than that ofGaN. Accordingly, in the semiconductor laser shown in FIG. 10, when thethickness of the (1100) LiTaO₃ substrate 201 is sufficiently larger thanthe total thickness of the n-Al_(z) Ga_(1-z) N cladding layer 203 to thep-Al_(z) Ga_(1-z) N cladding layer 207 formed by crystal growth and thegrowth temperature of each layer is as high as 1000° C., a strain in the(0001) direction (e_(xx)) of -0.44% and a strain in the (1120) direction(e_(yy)) of 1.6% are generated in the crystals of the layers 203 to 207when they are cooled to room temperature. In this way, anisotropicstrain can be generated in the plane of the In_(x) Ga_(1-x) N/GaNmultiple quantum well active layer 205. This greatly reduces the statedensity of the valence band and thus reduces the threshold current ofthe laser.

In the case where the total thickness of the crystal growth layers 203to 207 is large, the layers cannot bear the strain generated by thedifference in the thermal expansion coefficient between the layers andthe substrate, i.e., e_(xx) and e_(yy), This may cause dislocationdefect and thus reduces the strain. In such a case, as shown in FIG. 12,the LiTaO₃ substrate may be tilted by θ toward the (0001) direction fromthe (1100) direction and by φ toward the (1120) direction. By thistilting, the strain generated in the respective directions in thecrystal growth layers can be reduced to:

    e'.sub.xx =-0.4 cosθ

    e'.sub.yy =1.6 cosφ.

Accordingly, by appropriately selecting q and f, an occurrence of thedislocation defect can be prevented. The effect of preventing thedislocation defect is especially high by selecting q and f so thate'_(xx) +e'_(yy) =0.

In this example, LiTaO₃ was used for the substrate. However, othernonlinear optical crystal materials such as LiNbO₃, KTiOPO₄, KNbO₃, andLiB₆ 0₁₃ can also be used as long as they have large anisotropy in thethermal expansion coefficient and are stable in the growth temperature.

(EXAMPLE 3)

FIG. 13 is a sectional view of a wurtzite-type InGaN/AlGaN quantum wellsemiconductor laser of Example 3 according to the present invention.

Referring to FIG. 13, an AlN buffer layer 302, an n-Al_(z) Ga_(1-z) Ncladding layer 303, an Al_(y) Ga_(1-y) N first optical guide layer 304,an In_(x) Ga_(1-x) N/GaN multiple quantum well active layer 305, anAl_(y) Ga_(1-y) N second optical guide layer 306, a p-Al_(z) Ga_(1-z) Nfirst cladding layer 307, and a p-Al_(z), Ga_(1-z), N strain generatinglayer 308 are consecutively formed in this order on a (0001) sapphiresubstrate 301 by crystal growth. Then, the resultant structure is takenout from a crystal growth apparatus, and the p-Al_(z), Ga_(1-z), Nstrain generating layer 308 is shaped into a stripe with a width of 2 mmby etching. The resultant structure is placed in the crystal growthapparatus again, and a p-Al_(z) Ga_(1-z) N second cladding layer 309 isformed. An Si0₂ insulating film 310 is then formed over the top surfaceof the resultant structure. Openings 311 and 312 are formed at the SiO₂insulating film 310 for current injection. Finally, an anode electrode313 and a cathode electrode 314 are formed.

When the Al composition ratio z' of the p-Al_(z), Ga_(1-z), N straingenerating layer 308 is made larger than the Al composition ratio z ofthe p-Al_(z) Ga_(1-z) N first cladding layer 307 and the p-Al_(z)Ga_(1-z) N second cladding layer 309, the lattice constant of the formerbecomes smaller than that of the latter. As a result, compression straincan be generated in the surrounding crystals as shown in FIG. 14.

The above local strain can be generated because the width of thep-Al_(z), Ga_(1-z), N strain generating layer 308 is as small as about 2mm. If the width is larger, strain is only generated in the p-Al_(z),Ga_(1-z), N strain generating layer 308 itself, not to the surroundingcrystals. Since the p-Al_(z), Ga_(1-z), N strain generating layer 308 isof a stripe shape, strain is generated in the surrounding crystals inthe plane vertical to the stripe, while it is not in the plane parallelto the stripe. As a result, strain is generated only in the plane of theIn_(x) Ga_(1-x) N/Gan multiple quantum well active layer 305 vertical tothe stripe, causing anisotropy in the strain and thus reducing the holestate density. The strain in the In_(x) Ga_(1-x) N/GaN multiple quantumwell active layer 305 is greater as the multiple quantum well activelayer 305 is nearer to the p-Al_(z), Ga_(1-z), N strain generating layer308. Accordingly, the strain can be adjusted by setting the thickness ofthe p-Al_(z) Ga_(1-z) N first cladding layer 307 appropriately.

In this example, the Al composition ratio z' of the p-Al_(z), Ga_(1-z),N strain generating layer 308 was made larger than the Al compositionratio z of the p-Al_(z) Ga_(1-z) N first cladding layer 307 and thep-Al_(z) Ga_(1-z) N second cladding layer 309. Anisotropic strain canalso be generated when the former is made smaller than the latter. Inthis case, especially, an optical waveguide structure can be realized byuse of the p-Al_(z), Ga_(z-z), N strain generating layer 308, becausethe refractive index of the layer 308 is greater than that of theadjacent p-Al_(z) Ga_(z-z) N second cladding layer 309. Thus, arefractive index waveguide structure can be easily realized.

(EXAMPLE 4)

FIGS. 16A and 16B show a method for fabricating a semiconductor laseraccording to the present invention.

As shown in FIG. 16A, a chip of a semiconductor laser 401 which wasfabricated by forming an Al_(x) Ga_(y) In_(z) N (0≦x≦1, 0≦y≦1, 0≦z≦1)layer on the (0001) plane of a sapphire substrate by a crystal growthmethod such as MOVPE is mounted on a sub-mount 402 at a high temperatureof 200° C. As shown in FIG. 17, the sub-mount 402 includes an LiTaO₃dielectric 403 of anisotropic crystal and a solder member 404. Thesemiconductor laser 401 is mounted on a plane perpendicular to the(0001) plane, for example, on the (1120) or (1100) plane.

The solder member 404 is composed of Pb-Sn and the like, for example.The solder member melted at 200° C. is solidified when the temperaturelowers to room temperature, so that the semiconductor laser 401 issecured to the sub-mount 402. The thermal expansion coefficient of theLiTaO₃ dielectric 403 is 22×10⁻⁶ /K in the a-axis direction and 1.2×10⁻⁶/K in the c-axis direction. That is, the thermal expansions in thex-axis direction and the y-axis direction shown in FIG. 16A areconsiderably different from each other. As a result, non-uniform stressis applied to the semiconductor laser 401 when the semiconductor laser401 is secured to the sub-mount 402. The amount of the uniaxial stressapplied to the semiconductor laser 401 can be controlled by adjustingthe temperature to be increased. In other words, larger stress can beapplied as the temperature is higher.

The semiconductor laser 401 of this example uses wurtzite-type crystal,which can change the band structure of the valence band by applyinguniaxial stress in a direction vertical to the (0001) axis. This reducesthe effective mass and thus the state density. As a result, a highlyreliable semiconductor laser with reduced threshold current and drivingcurrent can be obtained.

Thus, the characteristics of the semiconductor light emitting elementcan be greatly improved by combining the wurtzite-type semiconductorlight emitting element with anisotropic crystal of which thermalexpansion coefficient varies depending on the direction.

In this example, the semiconductor laser 401 was mounted on a plane ofthe sub-mount vertical to the (0001) plane, for example, on the (1120)or (1100) plane. However, the plane direction is not limited to theabove as long as the sub-mount can provide uniaxial stress.

(EXAMPLE 5)

FIGS. 18A to 18B show another method for fabricating a semiconductorlaser according to the present invention.

As shown in FIG. 18A, a chip of a semiconductor laser 501 which wasfabricated by forming an Al_(x) GA_(y) In_(z) N (0≦x≦1, 0≦y≦1, 0≦z≦1)layer on the (0001) plane of a sapphire substrate by a crystal growthmethod such as MOVPE is mounted on a sub-mount 502 at a high temperatureof 180° C. As shown in FIG. 19, the sub-mount 502 includes an Fe-Nialloy 503, an Fe-Ni-Mn alloy 504, and a Pb-Sn solder member 505. TheFe-Ni alloy which is called Invar hardly changes its length as thetemperature changes. On the other hand, the Fe-Ni-Mn alloy exhibitssignificantly large thermal expansion as the temperature rises. Asub-mount which curves as the temperature changes can be obtained bylaminating the Fe-Ni alloy 503 and the Fe-Ni-Mn alloy 504.

The solder member melted at 180° C. is solidified when the temperaturelowers to room temperature, so that the semiconductor laser 501 issecured to the sub-mount 502. Non-uniform stress which is especiallylarge in one direction is applied to the semiconductor laser 501 whenthe semiconductor laser 501 is secured to the sub-mount 502. The amountof the uniaxial stress applied to the semiconductor laser 501 can becontrolled by adjusting the temperature to be increased.

The semiconductor laser 501 of this example uses the wurtzite-typecrystal, which can change the structure of the valence band by receivinguniaxial stress in a direction vertical to the (0001) axis. This reducesthe effective mass and thus the state density. As a result, a highlyreliable semiconductor laser with reduced threshold current and drivingcurrent can be obtained.

Thus, the characteristics of the semiconductor light emitting elementcan be greatly improved by combining the wurtzite-type semiconductorlight emitting element with the bimetal.

In this example, the sub-mount shown in FIG. 19 was used. Instead, theeffect of the present invention can be obtained by using any sub-mountwhich can be curved in one direction. For example, the structure shownin FIGS. 20A and 20B can also be used. In this structure, astripe-shaped Fe-Ni alloy 503 which does not expand with a temperaturechange, is formed on the bottom surface of an Fe-Ni-Mn alloy 504. Thesub-mount of this structure can be curved in a direction vertical to thestripe direction as shown in FIG. 20B.

FIG. 21 shows another method for fabricating a semiconductor laseraccording to the present invention.

As shown in FIG. 21, a chip of a semiconductor laser 551 which wasfabricated by forming an Al_(x) Ga_(y) In_(z) N (0≦x≦1, 0≦y≦1, 0≦z≦1)layer on the (1120) plane of a sapphire substrate by a crystal growthmethod such as MOVPE is mounted on a sub-mount 552. Ultraviolet(UV)-curable resin 554 is formed on the sub-mount 552. Then, thesemiconductor laser 551 is applied with stress from above by a collet553, while the UV-curable resin 554 is irradiated with UV light, so asto secure the semiconductor laser 551 to the sub-mount 552. Verticalstress is applied to the semiconductor laser 551 when the semiconductorlaser 551 is secured to the sub-mount 552.

The semiconductor laser 551 of this example uses the wurtzite-typecrystal, which can change the structure of the valence band by receivinguniaxial stress in a direction vertical to the (0001) axis. This reducesthe effective mass and thus the state density. As a result, a highlyreliable semiconductor laser with reduced threshold current and drivingcurrent can be obtained.

In the above alternative example, the stress was applied to thesemiconductor laser from above as shown in FIG. 21. Instead, the stresscan also be applied from the sides of the semiconductor laser as shownin FIG. 22. In this case, the semiconductor laser 551 is placed in aconcave portion formed on a sub-mount 555. A hexagonal-system compoundsemiconductor is used for the active layer of the semiconductor laser,and the crystal is grown in the c-axis direction.

As shown in FIG. 22, screws 557 for applying stress to the sides of thesemiconductor laser 551 via plate springs 556 are disposed on the sidesof the sub-mount 555. The semiconductor laser 551 is secured to theconcave portion with the UV-curable resin 554. By screwing the screws557, uniaxial strain is generated in the c plane of the active layer ofthe semiconductor laser 551. With the uniaxial strain, a semiconductorlaser with a small threshold current can be realized.

In the above alternative example, the semiconductor laser having the(1120) substrate was used. The effect of the present invention can beobtained for a structure where uniaxial stress is applied in a directionvertical to the (0001) plane.

The UV-curable resin was used in the above examples. Any other materialssuch as thermosetting resin can also be used as long as they can securethe semiconductor laser to the sub-mount.

(EXAMPLE 6)

FIGS. 23A to 23C show a method for fabricating an AlGaInN semiconductorlight emitting element according to the present invention.

The crystal growth in this method is conducted by low pressure MOVPE.Two MOVPE processes are required to fabricate the element. First, asshown in FIG. 23A, the first MOVPE process is conducted after degreasinga 6H-SiC substrate 601. The process will be described in detail.

Hydrogen gas is supplied in a reaction chamber of an MOVPE apparatus,and the pressure in the reaction chamber is set at 1/10 atmosphericpressure. Then, the temperature of the substrate 601 is raised up to1100° C. in the hydrogen gas atmosphere to clean the surface of the6H-SiC substrate 601.

After the temperature of the substrate 601 is lowered to 600° C.,ammonia gas as the V-group material and, after ten seconds, trimethylaluminum as the III-group material, are supplied to a surface of the6H-SiC substrate 601 so as to form a non-monocrystalline AlN layer 602with a thickness of 50 nm. The supply of trimethyl aluminum is thenstopped temporarily, in order to raise the substrate temperature to 900°C. Then, trimethyl aluminum as the III-group material is supplied again,so as to form a monocrystalline AlN layer 603 with a thickness of 5 μm.

Then, as shown in FIG. 23B, using SiO₂ as a mask for etching, two stripegrooves with a width of 3 μm are formed with a distance of 2 μmtherebetween.

After removing the mask for etching, the second MOVPE process isconducted in the following manner. Hydrogen gas is supplied in thereaction chamber of the MOVPE apparatus, and the pressure in thereaction chamber is set at 1/10 atmospheric pressure. Then, thetemperature of the 6H-SiC substrate 601 is raised to 1120° C. in anatmosphere which is a mixture of hydrogen gas and ammonia gas, so as toclean the surface of the substrate 601.

Then, as shown in FIG. 23C, after the temperature of the substrate 601is lowered to 1030° C., trimethyl aluminum, trimethyl indium, andtrimethyl gallium as the III-group materials are supplied, so as to forman Si-doped n-type AlGaInN cladding layer 604 with a thickness of 3 μm,an AlGaInN active layer 605 with a thickness of 20 μm, and an Mg-dopedp-type AlGaInN cladding layer 606 with a thickness of 2 μm over theentire top surface of the AlN layer 603 including the inside of thestripe grooves consecutively in this order. Finally, a p-side electrode607 and an n-side electrode 608 are formed to complete the laserstructure.

At the formation of the double-hetero structure in the above fabricationprocess, the efficiency by which each of the III-group elements isintroduced in the crystal in two stripe groove portions 609 is differentfrom that in a flat portion 610 between the two stripe grooves. As aresult, the composition varies, which corresponds to the variation inthe lattice constant. Accordingly, transverse stress is applied from thestripe groove portions 609 to the flat portion 610 which is sandwichedby crystals of a different lattice constant. This indicates that straincan be selectively applied to the flat portion in a direction verticalto the stripe direction. This substantially corresponds to uniaxialstrain in the plane of the active layer, which is effective in reducingthe state density of the valence band. Also, the active layer in theflat portion 610 between the two stripe grooves is curved. Thus, asemiconductor laser with a low threshold current and a stable transversemode can be realized by using the flat portion 610 as a light emittingportion.

The amount of strain applied to the flat portion 610 as the lightemitting portion can be easily controlled by changing the distancebetween the two stripe grooves, the depth of the stripe grooves, and thethickness of the Si-doped n-type AlGaInN cladding layer 604.

(EXAMPLE 7)

FIGS. 24A to 24C show another method for fabricating an AlGaInNsemiconductor light emitting element according to the present invention.The method of this example is different from the method in Example 6 inthat a concave portion is formed on a substrate instead of the twostripe grooves. Strain can be applied to an active layer of a flatportion on the bottom of the concave portion by using slope portions ofthe concave portion.

The crystal growth in this method is conducted by Vacuum MOVPE. TwoMOVPE processes are required to fabricate the element. As shown in FIG.24A, the first MOVPE process is conducted after degreasing a 6H-SiCsubstrate 651. The process will now be described in detail.

Hydrogen gas is supplied in a reaction chamber of an MOVPE apparatus,and the pressure in the reaction chamber is set at 1/10 atmosphericpressure. Then, the temperature of the substrate 651 is raised up to1120° C. in the hydrogen gas atmosphere to clean the surface of thesubstrate 651. After the temperature of the substrate 651 is lowered to600° C., ammonia gas as the V-group material and, after ten seconds,trimethyl aluminum as the III-group material are supplied to a surfaceof the 6H-SiC substrate 651, so as to form a non-monocrystalline AlNlayer 652 with a thickness of 50 nm. The supply of trimethyl aluminum isthen stopped temporarily, to raise the substrate temperature to 900° C.Then, trimethyl aluminum as the III-group material is supplied again, soas to form a monocrystalline AlN layer 653 with a thickness of 5 μm.

Then, as shown in FIG. 24B, using SiO₂ as a mask for etching, a stripegroove with a width of 3 μm is formed.

After removing the mask for etching, the second MOVPE process isconducted in the following manner. Hydrogen gas is supplied in thereaction chamber of the MOVPE apparatus, and the pressure in thereaction chamber is set at 1/10 atmospheric pressure. Then, thetemperature of the 6H-SiC substrate 651 is raised up to 1120° C. in anatmosphere of mixture of hydrogen gas and ammonia gas, so as to cleanthe surface of the substrate 651.

Then, as shown in FIG. 24C, after the temperature of the substrate 651is lowered to 1030° C., trimethyl aluminum, trimethyl indium, andtrimethyl gallium as the III-group material are supplied, so as to forman Si-doped n-type AlGaInN cladding layer 654 with a thickness of 3 μm,an AlGaInN active layer 655 with a thickness of 20 m, and an Mg-dopedp-type AlGaInN cladding layer 656 with a thickness of 2 μm over theentire top surface of the AlN layer 653 including the inside of thestripe groove consecutively in this order. Finally, a p-side electrode657 and an n-side electrode 658 are formed to complete the laserstructure.

At the formation of the double-hetero structure in the above fabricationprocess, the efficiency by which each of the III-group elements isintroduced in the crystal in stripe groove slope portions 659 isdifferent from that in a stripe groove flat portion 660. As a result,the composition varies, which corresponds to the variation in thelattice constant. Accordingly, transverse stress is applied from thestripe groove slope portions 659 to the stripe groove flat portion 660which is sandwiched by crystals of a different lattice constant. Thisindicates that strain can be selectively applied to the stripe grooveflat portion 660 only in a direction vertical to the stripe direction.That is, uniaxial strain can be applied in the plane of the active layerof the stripe groove flat portion 660. Thus, a semiconductor laser witha low threshold current and a stable transverse mode can be realized byusing the stripe groove flat portion 660 as a light emitting portion.

The amount of strain applied to the stripe groove flat portion 660 asthe light emitting portion can be easily controlled by changing thewidth and depth of the stripe groove and the thickness of the Si-dopedn-type AlGaInN cladding layer 654.

(EXAMPLE 8)

Referring to FIG. 25A, GaN crystal layers 802 and 803 are formed on a(0001) sapphire substrate 801 by MOVPE using trimethyl gallium (TMG) andammonia (NH₃). Hydrogen is used as a carrier gas. The growth pressure is100 Torr. At the vapor phase epitaxy, a portion of the substrate isselectively irradiated with a light beam emitted from an excimer laserand the like through a window formed in a reaction chamber. The crystalgrowth is conducted at 500° C., which is lower than the temperature atwhich monocrystal is normally obtained. This low temperature isnecessary for forming the GaN crystal layers with different latticeconstants arranged two-dimensionally.

FIG. 26 shows data by which the inventors have found that the latticeconstant of GaN crystal varies depending on the intensity of a laserbeam radiated during vapor phase epitaxy. This is considered to occurdue to the following reason: When the growth temperature is sufficientlylow, resultant GaN crystal is polycrystalline, which has a smallapparent lattice constant. If a laser beam with high intensity isradiated during the crystal growth, the temperature of the irradiatedportion selectively increases, resulting in monocrystalizing theirradiated portion. Referring to FIG. 25A, the (0001) sapphire substrate801 is selectively irradiated with an excimer laser beam with anintensity of 10 kW during the MOVPE process. As a result, the GaNcrystal layer 802 having a lattice constant normally obtained formonocrystal is formed in the laser beam irradiated region. On the otherhand, the polycrystalline GaN crystal layer 803 having a larger latticeconstant is formed in the region which is not irradiated with a laserbeam. As a result, as shown in FIG. 25B, a two-dimensionally anisotropicstrain state is realized in the boundary area of the GaN crystal layers802 and 803, where strain is generated along the boundary and no strainis generated in a direction perpendicular to the boundary. If this isapplied to an active layer of a gallium nitride semiconductor laser, forexample, significant improvement on characteristics thereof is expected.The growth temperature is not limited to 500° C., but a similar effectcan be obtained by 700° C. or less at which polycrystal is obtained.

(EXAMPLE 9)

In this example, GaN is further deposited on the GaN crystal layers 802and 803 of Example 8 by MOVPE at 1000° C. without irradiation with alaser beam. As a result, a GaN crystal layer 805 in FIG. 27 formed onthe polycrystalline GaN crystal layer 803 has a large lattice constant,while a GaN crystal layer 804 formed on the monocrystalline GaN crystallayer 802 has a small lattice constant. Important is that thetemperature for this crystal growth should be higher than thetemperature at which the GaN crystal layers 802 and 803 were formed.Using the polycrystalline GaN crystal layer 803 having a large latticeconstant as a buffer layer, the GaN crystal layer 805 which is moremonocrystalline and has good crystallinity can be formed. As a result, aGaN monocrystal layer with higher quality than that obtained by themethod only including laser beam irradiation as shown in FIG. 25A can berealized.

(EXAMPLE 10)

Information on lattice mismatching from the sapphire substrate can becontrolled by varying the thickness of the GaN crystal layers 802 and803 shown in FIG. 25A. Accordingly, by varying the thickness of the GaNcrystal layers 802 and 803, the lattice constant of the GaN crystallayer 805 can be varied and thus the amount of strain can be controlled.By using the GaN monocrystal layer having two-dimensional strainfabricated by the above method as an active layer of a gallium nitridesemiconductor laser, significant improvement on characteristics thereofis expected.

In the above examples, the growth of GaN monocrystal was described. Asimilar effect can also be obtained by using AlN, InN, or a mixturethereof. Also, a similar effect can be obtained by using a substratemade of SiC, ZnO, and the like, instead of the sapphire substratedescribed above.

(EXAMPLE 11)

FIGS. 28A to 28D show a fabrication process of Example 11 according tothe present invention. A sapphire substrate 1101 is placed in a reactionchamber of an MOVPE apparatus and heated to 1000° C. Then, the followinglayers are formed by MOVPE by supplying respective materials in thereaction chamber so as to form a double-hetero structure: an AlGaNcladding layer 1102 with a thickness of 5 μm by supplying hydrogen,ammonia, trimethyl aluminum, and trimethyl gallium; an n-type AlGaInNcladding layer 1103 with a thickness of 5 μm by supplying hydrogen,ammonia, trimethyl aluminum, trimethyl indium, and trimethyl gallium; anInGaN active layer 1104 with a thickness of 0.01 μm by supplyinghydrogen, ammonia, trimethyl indium, and trimethyl gallium; and a p-typeAlGaInN cladding layer 1105 with a thickness of 2 μm by supplyinghydrogen, ammonia, trimethyl aluminum, trimethyl indium, trimethylgallium, and diethyl zinc.

The AlInGaN cladding layers 1103 and 1105 and the InGaN active layer1104 have lattice constants larger than that of the AlGaN cladding layer1102. Accordingly, compression strain is generated in the resultantstructure.

Then, as shown in FIG. 28A, an insulating film 1106 is formed over thesides of the substrate 1101 and the layers 1102 to 1105 by thermal CVD.

The insulating film 1106 is selectively etched by photolithography andreactive ion etching with carbon tetrafluoride so that the side face ofthe AlGaN cladding layer 1102 is exposed as shown in FIG. 28B.

The AlGaN cladding layer 1102 is then etched from the exposed side faceby 5 μm by reactive ion beam etching with chlorine, forming thestructure as shown in FIG. 28C. The AlGaInN cladding layer 1103, theInGaN active layer 1104, and the AlGaInN cladding layer 1105 havelattice constants larger than the AlGaN cladding layer 1102 because theformer contain indium. Accordingly, the lattice constants of theselayers are reduced from their original values and compression strain isgenerated in the portion of these layers where the AlGaN cladding layer1102 exists underneath. On the contrary, in the other portion of theselayers where the AlGaN cladding layer 1102 has been removed by etching,no stress is applied and thus their original lattice constants remain.In FIG. 28C, therefore, these layers 1103 to 1105 are shown as expandingin the longitudinal direction.

FIG. 29 shows a sectional view of the structure after removing theinsulating film, together with a plane view thereof as is viewed fromabove. Regions 1107 and 1108 schematically show the lattice constants ofcrystals of the InGaN active layer 1104 as is viewed from above. Theregion 1107 of the InGaN active layer 1104 is located above the AlGaNcladding layer 1102 having a lattice constant smaller than that of theactive layer. Accordingly, the region 1107 of the InGaN active layer1104 receives stress from all the directions vertical to the growthdirection, generating two-dimensional compression strain. The region1107 expands in the growth direction by receiving the compressionstrain.

The region 1108 of the InGaN active layer 1104 below which the AlGaNcladding layer 1102 does not exist receives no stress in the rightdirection as is seen from FIG. 29 among the directions vertical to thegrowth direction. Accordingly, it is possible to apply compressionstrain in a selective direction.

Thus, strain in directions shown by arrows in FIG. 29 is generated inthe active layer 1104 at the boundary of the region 1107 below which theAlGaN cladding layer 1102 exists and the region 1108 below which thelayer 1102 does not exist. This strain corresponds to uniaxial strain inthe plane, reducing the state density of the valence band. In the region1107, the active layer 1104 receives compression strain isotropically inthe plane. This strain is therefore not effective in reducing thethreshold current. However, anisotropic strain is generated at theboundary of the regions 1107 and 1108, which is greatly effective inreducing the threshold current.

Alternatively, as shown in FIG. 28D, an InGaN cladding layer can be usedinstead of the AlGaN cladding layer 1102, and an AlGaInN active layercan be used instead of the InGaN active layer 1104. In this alternativeexample, also, tensile strain can be selectively applied to the AlGaInNactive layer at the boundary of the portion below which the InGaNcladding layer exists and the portion below which the cladding layerdoes not exist in a manner as described above.

The selective etching can be conducted by electrolysis, instead of thepatterning of the insulating film and the dry etching such as reactiveion beam etching as described above. In the etching by electrolysis, alayer to be etched is doped with impurities with high concentration soas to be etched faster than other layers, and a voltage is applied viaelectrodes in an electrolytic solution. An insulating layer with athickness of about 1 μm is required between the layer to be etched andother layers constituting the device structure to prevent electricalinterference.

(EXAMPLE 12)

FIGS. 30A to 30C schematically show a fabrication process of Example 12according to the present invention. A sapphire substrate 1201 is placedin a reaction chamber of an MOVPE apparatus and heated to 1000° C.Hydrogen, ammonia, and trimethyl gallium are supplied in the reactionchamber so as to form a GaN crystal nucleus 1202 on the substrate 1201.At this time, the pressure in the reaction chamber is set as low as 10Torr in order to obtain a low density for the formation of the growthnucleus.

Then, while the pressure is further lowered to 5 Torr, ammonia andtrimethyl gallium are supplied to grow GaN crystal 1203. This extremelylow pressure prevents a new crystal nucleus from being formed on thesubstrate and the crystal from being deposited vertically to thesubstrate. This is the condition where monocrystal is most easilydeposited on a crystal wall. Thus, the GaN crystal 1203 is grown aroundthe crystal nucleus 1202 spirally in parallel to the substrate, forminga spiral thin film.

FIG. 31 schematically shows a unit cell 1204 of the GaN crystal 1203.The unit cell 1204 does not receive stress in the radial direction, butreceives a tensile stress in the circumferential direction which isstronger as the portion thereof is closer to the outer circumference ofthe substrate. Accordingly, as long as dislocation does not occur, theGaN crystal 1203 is a crystal having asymmetric strain (anisotropicstrain) where tensile strain is generated only in the circumferentialdirection.

Thereafter, the pressure in the reaction chamber is raised to 80 Torr,to allow crystal to be deposited vertically to the substrate. Hydrogen,ammonia, trimethyl aluminum, and trimethyl gallium are supplied onto theGaN spiral thin film 1203, so as to form an n-type AlGaN cladding layer1205 with a thickness of 5 μm. Likewise, an InGaN active layer 1206 witha thickness of 0.01 μm is formed by supplying hydrogen, ammonia,trimethyl indium, and trimethyl gallium, and a p-type AlGaN claddinglayer 1207 with a thickness of 2 μm is formed by supplying hydrogen,ammonia, trimethyl aluminum, trimethyl gallium, and diethyl zinc. Thus,as shown in FIG. 30C, a double hetero structure is formed.

According to the method of this example, crystal with asymmetric straincan be easily formed only by varying the pressure in the crystal growthprocess without the necessity of the steps such as etching and selectivere-growth. As a result, a semiconductor laser with a small thresholdcurrent can be obtained by this method.

(EXAMPLE 13)

FIG. 34 is a schematic sectional view of a crystal growth apparatus usedin this example. Material gas is supplied from a gas inlet 1012 into areaction chamber 1011 made of quartz. A susceptor 1013 made of carbon isplaced in the reaction chamber 1011, on which a sample substrate 1014 ismounted. The susceptor 1013 is provided with a rotational mechanism forsecuring the in-plane uniformity of the composition and pressure of anepitaxial layer. The susceptor 1013 is heated by induction with a highfrequency coil 1015 disposed around the reaction chamber 1011. Athermocouple 1016 is disposed inside the susceptor 1013 for monitoringand controlling the temperature of the substrate. A gas outlet 1017,which is connected to a vacuum pump 1018, regulates the pressure in thereaction chamber and exhausts gas outside.

A crystal growth method using the above apparatus will be described withreference to FIGS. 34 and 35A to 35D. First, a (0001) plane α-Al₂ O₃sapphire substrate 1031 of which surface has been cleaned with chemicaltreatment with an organic solvent and a hydrochloric acid groupsubstance and by washing with pure water is mounted on the susceptor1013 and secured thereto with a holder 1021. High-purity hydrogen gaspurified by a purifying apparatus is supplied from the gas inlet 1012and replaces air in the reaction chamber 1011. After the supply of thehydrogen gas for several minutes, the vacuum pump 1018 is driven to setthe pressure in the reaction chamber at 10 Torr. Following thestabilization of the pressure, the susceptor 1013 is heated by inductionwith the high frequency coil 1015, so as to raise the temperature of thesample substrate 1014 to 1200° C. This temperature is kept for about 10minutes, to clean the surface of the substrate.

The temperature of the substrate is lowered to 400° C. Then, TMG(trimethyl gallium) and NH₃ (ammonia) as the material gas are suppliedfrom the gas inlet 1012, so as to form an amorphous GaN film 1035 with athickness of 0.1 μm as shown in FIG. 35A. At this time, since thesubstrate temperature is low compared with a normal growth condition,the decomposition efficiency of NH₃ is low. In consideration of this,the flow rate ratio of NH₃ to TMG is set at 10000:1. If the substratetemperature at the crystal growth is higher than the above temperature,crystal grows three-dimensionally, i.e., hexagon-pole shaped crystalsare grown like islands, failing in obtaining a uniform amorphous GaNfilm.

The sample substrate is taken out from the reaction chamber 1011 afterthe temperature thereof is lowered. The amorphous GaN film 1035 is thenetched by photolithography to form stripes in a direction crossing the Rplane of the sapphire substrate 1031 as shown in FIG. 35B. The R planeof the sapphire substrate 1031 is a plane shown as the section in FIGS.35A to 35D. The stripes extend in a direction vertical to the plane ofthese figures, i.e., crossing the R plane. The width of the stripes andthe distance between the stripes are 5 μm and 50 μm, respectively.

The sample substrate 1014 is placed again in the reaction chamber 1011after being washed sufficiently with pure water. NH₃ gas is supplied inthe reaction chamber this time, instead of hydrogen gas, and the samplesubstrate 1014 is heated to 1120° C. as in the manner described above,so as to clean the surface of the sample substrate.

Then, GaN films are formed by a normal two-stage epitaxy by supplyingTMG and NH₃ from the gas inlet 1012. More specifically, first, thesubstrate temperature is lowered to 600° C. to facilitate a GaN film1033 with a thickness of 0.05 μm to be formed three-dimensionally, i.e.,hexagon-pole shaped crystals to be grown like islands as shown in FIG.35C. Then, the substrate temperature is raised to 1050° C. to form a GaNfilm 1034 with a thickness of 5.0 μm by epitaxy as shown in FIG. 35D.The flow rate ratio of NH₃ to TMG is 300:1. The portions of the GaN film1034 formed on the stripes of the amorphous GaN film 1035 are amorphousbecause only amorphous crystal is grown on an amorphous crystal. Theamorphous portions of the GaN film 1034 are specifically calledamorphous GaN films 1036.

The other portions of the GaN film 1034 interposed between the amorphousGaN films 1036 constitute element formation regions 1041. In theseregions, the dislocation density is low and the strain in the crystal isanisotropic. In other words, strain caused by the difference in thelattice constant between the substrate 1031 and the GaN film 1034 ismaintained in a direction parallel to the stripes, while it is minimizedin a direction perpendicular to the stripes.

Thus, if a semiconductor laser, for example, is formed in the elementformation region 1041 having anisotropic strain, the threshold currentthereof can be reduced due to the anisotropic strain in the region.

Now, the crystal quality of the epitaxial layers obtained by thisexample will be described. For comparison, FIG. 36 shows a sectionalview of GaN epitaxial layers 1033 and 1034 with a thickness of 5 μmformed on a (0001) plane α-Al₂ O₃ sapphire substrate 1031 by aconventional two-stage epitaxial method. This exhibits a distribution ofdislocation observed by a transmission electron microscope.

Dislocations 1032 generated uniformly due to strain caused by latticemismatching extend from an interface 1037 between the substrate 1031 andthe GaN epitaxial layer 1033 toward the surface of the epitaxial layerswhile meandering. The dislocations which disappear or come out midwayshow that they extend in the direction vertical to the plane of thefigure, not indicating that they are distinguished. The dislocationdensity estimated from the image obtained by the transmission electronmicroscope is 10⁹ /cm² or more, and the distribution is uniform. Thelattice strain applied to the epitaxial layers is isotropic in theplane.

The epitaxially formed GaN films of this example will now be describedwith reference to FIG. 37. The distribution of dislocations observed bythe transmission electron microscope is also shown in FIG. 37schematically. It is observed from FIG. 37 that a considerably largenumber of dislocations reach the portions above the stripes of theamorphous GaN film 1035 where crystal defects are concentrated. Thisoccurs within the region of the GaN films with a thickness of 3 μm. Inother words, while strain is minimized in a direction 1039 vertical tothe stripes, strain tends to remain in the crystal in a direction 1040parallel to the stripes. As a result, epitaxial films where strain dueto lattice mismatching is mostly remained in the direction 1040 parallelto the stripes are obtained.

Other effects are as follows. The dislocation density of the elementformation regions 1041 interposed between the stripes is 10⁵ /cm² orless. This indicates that the GaN films in these regions have excellentcrystallinity compared with the comparative example shown in FIG. 36.Also, the strain in the direction parallel to the stripes is differentlygenerated from that in the direction vertical to the stripes. That is,the element formation regions 1041 have anisotropic strain.

In this example, the stripes of the amorphous GaN film 1035 were formedon the substrate. A similar effect can also be obtained by using oxidefilms and nitride films such as SiO₂ and SiN.

FIG. 38 shows the case of using an SiO₂ film. GaN films 1033 and 1034are selectively formed on a substrate on which strips of an SiO₂ film1038 with a thickness of 0.1 μm have been formed. In this case, as inthe above case, epitaxial films where strain due to lattice mismatchingmostly remains in a direction parallel to the stripes are obtained.Since the GaN films are not formed on the stripes of the SiO₂ film(selective growth), the GaN films are formed like islands. In theisland-like GaN film 1034, strain remains in the direction parallel tothe stripes while being minimized in the direction vertical to thestripes, realizing anisotropic strain.

In this example, the GaN epitaxy on the (0001) plane α-Al₂ O₃ sapphiresubstrate was described. The present invention is not limited to theabove case, but can be applied to any epitaxy causing latticemismatching, providing effects similar to the above.

In this example, a plurality of stripes of the GaN film 1035 wereformed. However, with at least one stripe, anisotropic strain can begenerated in the element formation region of the GaN film. This isbecause anisotropic strain always exists near the stripe.

The two-stage epitaxial method was used to form the GaN films on thesubstrate. However, the GaN film 1034 may be directly formed on thesubstrate without forming the GaN film 1033.

Thus, it has been verified that, in the epitaxy causing latticemismatching, the method according to the present invention is effectivein obtaining an epitaxial film where lattice strain generated due to thelattice mismatching can be concentrated in a specific direction.

(EXAMPLE 14)

FIGS. 32A and 32B show a semiconductor light emitting device of Example14 according to the present invention. In this example, stress isphysically applied to the sides of a semiconductor light emittingelement by use of a shape memory alloy so as to generate strain in anactive layer.

Referring to FIG. 32A, a concave portion 1403 is formed on a Cu-Ni-Alshape memory alloy 1404. The width of the concave portion 1403 is 480μm, which is slightly smaller than the width of a semiconductor lightemitting element 1402 to be described later. The surfaces of the concaveportion 1403 are lightly treated for insulation to prevent thesemiconductor light emitting element 1402 from being short-circuitedwhen the element is placed in the concave portion 1403.

First, the concave portion 1403 is mechanically enlarged, and thesemiconductor light emitting element 1402 is placed in the concaveportion 1403. The resultant structure is placed in a heating chamber andheated to 80° C. The shape memory alloy is thus heated and resumes theoriginal shape. As a result, stress is applied to the semiconductorlight emitting element 1402 in the x direction vertical to the stripe.This makes it possible to generate strain uniaxially (in the xdirection) in the c plane of an active layer, and thus reduce thethreshold current of the light emitting element.

Since the size of the concave portion 1403 of the shape memory alloy ispredetermined, the stress applied to the light emitting element isdetermined depending on the size of the concave portion 1403. Theconcave portion 1403 enlarged to receive the light emitting elementresumes the original shape only by heating. Accordingly, the mounting ofthe light emitting element is easy. The resultant structure with thelight emitting element fitted in the shape memory alloy is asemiconductor light emitting device 1401.

The semiconductor light emitting element 1402 is fabricated by MOVPE inthe following manner.

First, a well cleaned (0001) sapphire substrate (c plane) is placed on asusceptor in a reaction chamber. After a hydrogen atmosphere isestablished in the reaction chamber, the substrate is heated to 1080° C.to clean the substrate.

The substrate is then cooled to 505° C. Four liters/min. of ammonia and30×10⁻⁶ mols/min. of trimethyl gallium as the material gas and 2liters/min. of hydrogen as the carrier gas are supplied, so as to form aGaN buffer layer on the substrate.

The supply of trimethyl gallium is then stopped. The substratetemperature is raised to 1080° C. Then, 50×10⁻⁶ mols/min. of trimethylgallium and 2×10⁻⁹ mols/min. of silane gas are supplied, so as to form asilicon-doped n-type GaN layer.

Then, the supply of the material gas is stopped. The substratetemperature is lowered to 800° C. The carrier gas is switched fromhydrogen to nitrogen. Then, 2 liters/min. of nitrogen and 2×10⁻⁶mols/min. of trimethyl gallium, 1×10⁻⁵ mols/min. of trimethyl indium,2×10⁻⁶ mols/min. of diethyl cadmium, and 4 liters/min. of ammonia as thematerial gas are supplied, so as to form a cadmium-doped In₀.14 Ga₀.86 Nlayer.

The supply of the material gas is then stopped. The substratetemperature is raised to 1080° C. Then, 50×10⁻⁶ mols/min. of trimethylgallium, 3.6×10⁻⁶ mols/min. of cyclopentadienyl magnesium, and 4liters/min. of ammonia are supplied, so as to form a p-type GaN layer.

The p-type GaN layer and the n-type InGaN layer of the semiconductorlight emitting element are partly etched to expose the n-type GaN layer.P-type and n-type ohmic electrodes are formed on the p-type GaN layerand the n-type GaN layer, respectively. The semiconductor light emittingdevice of this example is obtained by mounting the thus-fabricatedsemiconductor light emitting element.

(EXAMPLE 15)

FIGS. 33A and 33B show a semiconductor light emitting device using thesemiconductor light emitting element shown in Example 14. Stress ismechanically applied to the light emitting element from the sidesthereof.

Referring to FIG. 33A, the semiconductor light emitting element isformed by growing AlGaInN crystal which is a hexagonal-system compoundsemiconductor on the (0001) plane (c plane). Accordingly, stress isapplied to the semiconductor light emitting element from the sidesthereof so as to generate strain uniaxially (in the y direction) in thec plane.

A semiconductor light emitting element 1502 is placed in a vessel 1503for stress application, and stress is gradually applied to thesemiconductor light emitting element 1502 from the sides thereof. Themagnitude of the stress is adjustable by turning a handle 1504.

In FIG. 33A, the (0001) plane sapphire substrate is used for thesubstrate of the semiconductor light emitting element 1502. Instead, asshown in FIG. 33B, by using the R plane for the substrate and growingAlGaInN crystal on the substrate, stress can be applied in a directionvertical to the substrate (y direction). This is because, though thestate density of the valence band cannot be reduced by generating strainin the c-axis direction, it can be drastically reduced by generatinganisotropic strain in the c plane. Using the R plane for the substrate,the c axis is directed as shown in FIG. 33B. Accordingly, anisotropicstrain can be generated in the c plane when stress is applied in thedirection vertical to the substrate (y direction). Naturally, asdescribed above with reference to FIG. 33A, anisotropic strain can alsobe generated in the c plane by applying stress from the sides of thesemiconductor light emitting element grown on the R plane.

Thus, according to the method of this example, anisotropic strain can bemechanically generated in the c plane of the semiconductor lightemitting element. Accordingly, the state density of the valence band canbe reduced, and thus the threshold current for laser oscillation can bedrastically reduced.

According to the present invention, based on the fact that, in the caseof applying anisotropic strain in the c plane of a hexagonal-systemcompound semiconductor, the effective mass of holes near the top of thevalence band is lowered, a semiconductor light emitting element with areduced threshold current can be realized by generating anisotropicstrain in the c plane of an active layer composed of a hexagonal-systemcompound semiconductor grown in the c-axis direction.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A semiconductor laser comprising an active layerformed in a c-axis direction, wherein the active layer is made of ahexagonal-system compound semiconductor, and anisotropic strain isgenerated in a c plane of the active layer.
 2. A method for fabricatinga semiconductor laser comprising the step of forming an active layermade of a hexagonal-system compound semiconductor in a c-axis direction,wherein the active layer is formed so that anisotropic strain isgenerated in a c plane.
 3. A semiconductor laser according to claim 1,wherein the hexagonal-system compound semiconductor is of a wurtzitetype.
 4. A semiconductor laser according to claim 1, wherein thehexagonal-system compound semiconductor is of a 4H type.
 5. Asemiconductor laser according to claim 1, wherein the hexagonal-systemcompound semiconductor is of a 6H type.
 6. A semiconductor laseraccording to claim 1, wherein the anisotropic strain is uniaxial strain.7. A semiconductor laser according to claim 1, wherein the anisotropicstrain is shearing strain.